1. Field of the Invention
The present invention relates, in general, to halogen lamps. In particular, the invention relates to a double bulb type halogen lamp wherein an optical interference film composed of a selected metal oxide substance is coated on at least either the outer surface of an inner bulb thereof or the inner surface of an outer bulb to transmit visible rays and to reflect infrared rays from a light source thereof.
2. Description of the Related Art
A double bulb type halogen lamp in which a halogen lamp is airtightly enveloped by an outer bulb is used as a reflector lamp, e.g., a sealed beam lamp, to achieve a high output. A prescribed amount of halogen element, e.g., iodine, bromine, etc., or a compound thereof is sealed in the halogen lamp. The temperature of the inner bulb of the halogen lamp greatly increases during the operation to perform a halogen cycle. In recent years, an improved double bulb type halogen lamp has been developed to achieve a high efficiency and to radiate light having a small amount of infrared rays, i.e., a so-called cool beam. In such an improved double bulb type halogen lamp, an optical interference film is formed on the outer surface of the inner bulb to transmit visible rays and to reflect infrared rays. Furthermore, air in a sealed space between inner and outer bulbs is exhausted, or a prescribed amount of inert gas is sealed in the sealed space.
The above-described optical interference film includes a high refractive index layer made of a metal oxide substance, e.g., titanium oxide (TiO.sub.2), tantalum oxide (TaO.sub.2), or zirconium oxide (ZrO.sub.2), and a low refractive index layer made of other metal oxide substance, e.g., silicon oxide (SiO.sub.2), or selenium oxide (SeO.sub.2), alternately stacked one to the other at six to twenty one layers on the bulb. The light from a specific wavelength range may be transmitted or reflected by controlling thickness of each refractive index layer. When the optical interference film transmits visible rays and reflects infrared rays in light radiated from the light source, i.e., a coil filament, infrared rays reflected by the interference film returns toward the filament and heats up the filament, resulting in a high luminous efficiency in halogen lamp. However, since the heat discharge from the inner bulb of the double bulb type halogen lamp is not effective, the temperature of the inner bulb increases in excess during the operation. The temperature of the base portion of the halogen lamp also increases close to that of the inner tube.
In the above-described conventional double bulb type halogen lamp, the temperature of the optical interference film reaches 500.degree..about.800.degree. C. during the operation. The optical interference film becomes black as the temperature of the optical interference film frequently increases to such a high temperature during an extended operation period, and thus, the optical interference film loses the original optical characteristics. The light output of the halogen lamp is greatly reduced.